This invention relates to an apparatus and method for casting metallic alloys. More particularly, the alloy is delivered to a mold as partially solidified droplets reducing the development of coarse dendrites.
In a conventional metal casting process, molten metal is delivered to a water cooled mold and solidifies by heat extraction through the surfaces of the mold. During solidification, dendritic growth occurs in certain alloy compositions. The dendrites grow from the mold walls and extend towards the center of the casting. Dendritic branching produces a three dimensional solid web. The dendritic web inhibits the flow of molten metal from the center of the mold to the solidification front. As a result, castings with significant porosity are produced. This type of directional dendritic solidification can also lead to hot tears.
One solution, disclosed in U.S. Pat. No. 4,577,676 to Watson, is reheating a portion of the mold subsequent to the formation of the dendrites. The dendrites detach from the mold and are remixed into the melt. The dendrites then serve as nuclei for grain refinement as the melt solidifies into a cast ingot.
Another method is disclosed in U.S. Pat. No. 4,972,899 to Tungatt. A feed tube separates a molten metal source from a mold. The feed tube is cooled by cyclically flowing cooling fluid. As the melt solidifies, a zone of fine dendrites is formed on the inner surface of the mold. An inductor reheats the zone of fine dendrites which then detach falling back into the melt. The dendrites serve as nuclei for grain refinement as the melt solidifies into a cast ingot.
One way to reduce dendritic growth is spray casting. Spray casting, as described in U.S. Pat. Nos. 3,826,301 and 3,909,921, both to Brooks and both incorporated in their entirety by reference herein, is the rapid solidification of metal into shaped preforms by means of an integrated gas atomizing/spray deposition process. A controlled stream of molten metal is delivered to a gas atomizer where high velocity jets of gas atomize the stream. The resulting spray of metal particles is directed onto a collector where the hot particles coalesce to form a dense preform. The preform can then be further processed, typically by hot working, to form a semi-finished or finished product.
Spray casting has been used to form alloys having a finer dispersion of intermetallics than is possible by conventional casting as disclosed in U.S. Pat. No. 5,074,933 to Ashok et al. Intermetallic growth is confined within the individual droplets of atomized metal, preventing the formation of a coarse intermetallic phase.
In conventional spray casting, the droplets are partially solidified or supercooled prior to impact with the collector. Solidification is rapidly completed following impact. The droplets are predominantly solid at the time of impact and the deposit has a high viscosity. As a result, gas pores are retained within the deposit. A second issue with conventional spray casting is overspray. About 20% of the droplets miss the collector and become powder scrap.
In conventional spray casting, predominantly solid droplets impact the collector. U.S. Pat. No. 5,131,451 to Ashok discloses formation of a metallic strip by spray casting onto a continuous belt. To ensure good metal flow across the belt, the droplets are at least 50% liquid. This method is particularly useful for casting metal strip. The method is limited to horizontal casting and the gas pressure and droplet velocity must be sufficiently low to minimize splashing. Turbulence generated by the atomized droplets striking the solidifying surface of the thin strip can cause shape control problems and macro-defects.